HYDRO-FLATTENING

Sanborn has developed a custom workflow to produce hydro-flattened and or hydro-enforced DEMs per the specifications outlined in the USGS Lidar Base Specification Guidelines. “Hydro-flattened” describes the specific type of DEM required by the USGS National Geospatial Program (NGP) for integration into the NED.

Hydro-flattening is the process of creating a Lidar-derived DEM in which water surfaces appear / behave as they would in traditional topographic DEMs created from photogrammetric digital terrain models (DTMs). A hydro-flattened DEM is a topographic DEM and should not be confused with hydro-enforced or hydro conditioned DEMs, which represent hydrologic surfaces.

A traditional topographic DEM (such as NED) represents the actual ground surface, in which hydrologic features are handled in established ways. Roadways crossing drainages / passing through culverts remain in the surface model because they are part of the landscape. This is because the culvert beneath the road is the man-made feature. In addition, bridges and other man-made structures above the landscape are removed.

Hydro-enforcement is a modification of the traditional topographic DEM (Hydro-flattened DEM) that produces hydrologic surfaces that are fundamentally different at a functional level. Hydrologic surfaces are identical to topographic surfaces in many respects, but differ significantly from a topographic DEM (where roadways over culverts are included in the surface as part of the landscape). From the hydrologic perspective, these roadways create artificial impediments (digital “dams”) to the drainages and introduce sinks (un-drained areas) into the landscape.

Sanborn digitizes hydrology features and edge-of-water breaklines using stereo-intensity images in a 3D dataset. Planimetric maps will include all hydrology and select man-made features (such as bridges and dams) that affect hydrological flow. During the planimetric phase, each tile is viewed in both 2D and 3D perspectives as an intensity image, shaded relief map and / or digital surface model (DSM). We then populate the “Z” value for the points, lines, and polygons referencing the bare-earth Lidar. Each tile’s hydrology feature is digitized in true horizontal position, and the Feature Type and Class are captured along with X, Y, and Z attributes. Once all features have been captured, a second technician reviews the imagery for content and accuracy, correlating the collected features with the base Lidar data and reference imagery before releasing the tile for final edits.

Sanborn Hydro Flattening Deliverables

Sanborn will deliver hydro-enforced / hydro-flattened data in accordance with the following parameters as defined in the USGS Lidar Base Specification Guidelines.

Inland ponds and lakes

Water bodies of 8,000 m2 (2 acres) or greater surface area at the time of collection are flattened.

Flattened water bodies are represented as flat and level water surface (a single elevation for every bank vertex defining the water body’s perimeter).

The entire water-surface edge is at or below the immediately surrounding terrain

Long impoundments, such as reservoirs, inlets, and fjords, whose water-surface elevations decrease with downstream travel, are compiled as streams or rivers.

Inland streams and rivers

Streams and rivers of a 30-m (100-ft) nominal width are flattened.

Streams or rivers whose width varies above and below 30 meters will not be broken into multiple segments.

Flattened streams and rivers are represented as a flat and level water surface from bank to bank, perpendicular to the apparent flow centerline.

Flattened streams and rivers are represented as a gradient downhill water surface, following the immediately surrounding terrain.

In cases of sharp turns of rapidly moving water, where the natural water surface is notably not level bank-to-bank, the water surface is represented as it exists while maintaining an aesthetic cartographic appearance.

The entire water surface edges are at or below the immediately surrounding terrain.

When the identification of a structure as a bridge or culvert cannot be made definitively, the feature is regarded as a culvert.

Non-tidal boundary waters

Boundary waters, regardless of size, are represented only as an edge or edges within the project. Collection does not include the opposite shore.

The entire water surface edges are at or below the immediately surrounding terrain.

The water surface elevation will be consistent throughout the project.

The water surfaces are flat and level, or as appropriate for the type of water body (level for lakes, a gradient for streams and rivers).

Any unusual changes in the water surface elevation during the course of the collection (such as increased upstream dam discharge) are documented in the project metadata.

In the event of an unusual change in water surface elevation, the water body is handled as described in Tidal Waters (below).

Tidal waters

Tidal water bodies are defined as any water body that is affected by tidal variations, including oceans, seas, gulfs, bays, inlets, salt marshes, and large lakes. Tidal variations during data collection (or between different data collections) will result in lateral and vertical discontinuities along shorelines. As per USGS Lidar Base Specification Guidelines, acquisitions are planned to minimize tidal differences at the land-water interface (if possible). In addition to meeting the requirements for inland water bodies (listed in the Inland Ponds, Lakes, Inland Streams, and Rivers sections above) when appropriate, the tidal water bodies are compiled to meet the following requirements:

Within each water body, the water surfaces are flat and level for each different water surface elevation.

Vertical discontinuities within a water body resulting from tidal variations during the collection are considered normal and are retained in the final DEM.

Horizontal discontinuities along the shoreline of a water body resulting from tidal variations during the collection are considered normal and are retained in the final DEM.

Islands

Permanent islands of 4,000 sq m (1 acre) or larger shall be delineated in all water bodies.

Final processing includes the use of custom routines to validate flow direction and monotonicity to ensure all vector nodes are flowing downhill for single- and double-line streams, or are the same elevation for pools of water, including lakes and ponds. Other processing may include using standard Esri tools to eliminate pseudo-nodes that would impair hydrologic analysis. These added-value processing techniques provide a gateway for the datasets to be used for redefining and providing better resolution and accuracy for watershed, lake shed, and boundaries definition.

For more information or to order Sanborn products or services, call customer service at 1.866.SANBORN (1.866.726.2676), or contact us by e-mail at information@sanborn.com.

For more information or to order Hydro Flattening Products, call Sanborn customer service at 1.866.SANBORN (1.866.726.2676), or contact us by e-mail at information@sanborn.com.

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